On the horizon

The Amazon's ancient flow reversal

Graduate student Russell Mapes's study started with a simple question: How fast does silt move from the headwaters of the Amazon River high in the Andes to the vast delta that spills into the Atlantic? But he got a bit sidetracked after discovering that grains in rocks from the central Amazon basin were too old to have moved down from the Andes.

The results indicate that the Amazon once flowed east to west, tumbling out of mountains that formed when South America and Africa scraped past each other between 145 million and 65 million years ago. Previous studies from a handful of locations along the river pointed to the possibility that the Amazon had been thrown into reverse. Mr. Mapes; his adviser, Drew Coleman, from the University of North Carolina; and colleagues in Brazil took their samples along some 80 percent of the river's run. These samples clearly indicated the switch in flow.

It took the erosion of the eastern mountains and the rise of the Andes to throw the river into reverse. The team reported its results Tuesday at the Geological Society of America's annual meeting in Philadelphia.

The bays along the Gulf Coasts of Texas and Louisiana have been dancing a slow-motion rumba for millennia – with the bays expanding, contracting, and encroaching farther inland, only to shift close to the sea again – several times.

These results from core samples taken on coastal bays and their environs suggest that, over this century, coastal communities there are likely to face more flooding, says John Anderson, a University of Houston oceanographer who did the research. Sea-level rise and a relative dearth of silt coming down rivers are the likely triggers, he says.

His research found that during the past 10,000 years, each of seven bays and coastal lakes have experienced an average of six flooding events. Because the floods don't match up to sea-level trends, he concludes that the floods resulted from a lack of sediment coming down rivers that feed the bays and lakes. Without this replenishment, bays expanded by up to 30 percent over the course of 100 to 200 years. And they extended their reach tens of miles inland. The process reversed itself when sediments once again started to flow and fill in the bays.

Given projected sea-level rise over this century, as well as the large amounts of sediment rapidly backing up against dams upriver, Mr. Anderson says, there's ample evidence that these one-two punches will occur over the next 100 years in each of the bays he's studied. He reported his results Monday at the Geological Society of America's annual meeting.

The lowly lamprey is nothing if not a survivor. The parasitic fish is one of only two vertebrate species without a jaw. This suggests that it is one of the most primitive forms of vertebrates. Now, paleontologists have discovered a 360-million-year-old ancestor to the modern lamprey, the oldest lamprey yet found. Until now, the oldest lamprey fossil dated back 125 million years.

What's striking about the new find, researchers say, is how little lampreys have changed over that time, surviving four major extinction events in the process. This makes the animal a useful reference for tracing evolutionary changes in other vertebrates. Subtle variations in the overall size of its mouth, and the mouth's size relative to the rest of its body, are the biggest differences between the fossil specimen and its modern descendants, the researchers say.

Paleontologists from the University of Chicago and the University of Witwatersrand in South Africa unearthed the fossil some 18 months ago from rock formations that once formed the mouth of an ancient river. Lamprey fossils are extremely rare, the scientists say, because the creatures lack a skeleton or an appreciable backbone. Its soft tissue decomposes quickly. In this case, however, the find was remarkably preserved, the team reports in Thursday's issue of the journal Nature.